Heterostructural one-unit-cell FeSe/SrTiO₃: from high-temperature superconductivity to topological states

Abstract

High-temperature superconductivity in one-unit-cell (1-UC) FeSe/SrTiO 3 heterostructure has become a research frontier in condensed-matter physics and material science. The superconducting transition temperature (T c ) of ultrathin FeSe film is significantly enhanced compared to its bulk counterpart and possibly approaches the liquid-nitrogen region according to in situ spectroscopic measurements. Particularly, the Fermi-surface topology of 1-UC FeSe consists of no hole pockets at Brillouin-zone center, which poses a great challenge to the well-accepted s ± -wave pairing nesting the sign-different electron-hole Fermi pockets in iron-based superconductors. In this review, we present the explorations of T c enhancement, electron pairing and topological phases in 1-UC FeSe/SrTiO 3 . The potential coexistence of high-temperature superconductivity and topological electronic states promotes such two-dimensional heterostructure as the candidate of next-generation connate high-temperature topological superconductor and/or topological-quantum-computation platform.

On the Remarkable Superconductivity of FeSe and Its Close Cousins

“…FeSe with excess Fe was discovered to be an 8 K superconductor in 2008 [61], but a cold vapor deposition technique was required to reliably make high-quality, stoichiometric, crystals [60]. A summary of interesting properties of this compound has been provided in earlier reviews [24,62,63], reviews on monolayer FeSe can be found in references [64][65][66][67]. As for FeTe, it turns out to be the most stable compound of the 11 chalcogenides in its pristine form [22], which may, together with the presence of interstitial Fe, be responsible for the difficulty of making homogeneous samples doped with Se away from the FeTe point.…”

“…In the resulting band structure, the inverted parity-exchanged hole (electron) bands acquire p z /d xy (d xz /d yz ) orbital weight. For further details about the generations of topological bands in FeSCs monolayers, we refer to references [261,376] and recent reviews in references [25,67].…”

Orbital-Selective High-Temperature Cooper Pairing Developed in the Two-Dimensional Limit

“…Previously, orbitally selective pairing was discovered only in bulk FeSe, while the 2D-limit counterpart, one-unit-cell (1-UC) FeSe, , has been scarcely explored (section I in the Supporting Information). For 1-UC FeSe/SrTiO 3 (001), although the significantly high T c of 55–65 K has encouraged extensive investigations, the central pairing issues remain debated − because of absent Γ-hole pockets. Concretely, the pairing is controversial mainly over s ++ -, incipient s ± -, extended s ± -, and nodeless d-wave, , partially suffering from the experimental challenge of effectively distinguishing different bosonic modes (e.g., spin fluctuations vs phonon) for mediating the coherent Cooper pairs.…”

Identifying $s$-wave pairing symmetry in single-layer FeSe from topologically trivial edge states

“…Moreover, the behavior of Caroli-de Gennes-Matricon states in the vortex core disfavors d-wave pairing [16,17]. On the other hand, the controversy over the two leading contenders, i.e., sign-preserving sand sign-changed s ± -wave, remains unresolved, despite extensive impurity-scattering investigations [18][19][20][21][22][23][24] including measurements employing the recently developed quasiparticle interference technique [18][19][20][25][26][27]. The conflict situation stems from the difficulties in proving the practical magnetic or nonmagnetic nature of the impurities under investigation [21] and distinguishing bound states under weak scattering potential [28].…”